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(a) The limiting reactant is the reactant in a chemical reaction that is completely consumed when the reaction reaches completion, determining the maximum amount of product that can be formed. The excess reactant is the reactant that is not completely used up in the reaction and is present in a higher amount compared to the stoichiometric ratio needed. (b) The amounts of products formed in a reaction are determined only by the amount of the limiting reactant because once the limiting reactant is depleted, the reaction stops, even if there are excess reactants left, as there is nothing left for the excess reactants to react with. (c) The choice of limiting reactant should be based on its number of initial moles, not on its initial mass in grams because the stoichiometry of a reaction is based on a mole-to-mole ratio of reactants and products, not on mass ratios. Comparing the number of moles of each reactant present with their stoichiometric ratio helps determine which reactant will be used up first. Using mass instead of moles could lead to incorrect conclusions about the limiting reactant, as it doesn't take into account the different molar masses of the compounds involved in the reaction.